1. Field of the Invention
The present invention relates to a gas sensor including a detection element configured to detect a specified gas component contained in a gas to be detected. An example of the gas sensor is one having a stacked detection element made by simultaneously sintering a plurality of stacked layers which contain different main components.
2. Description of the Related Art
The use of a gas sensor attached to an exhaust pipe of an automobile and including a detection element configured to vary an electromotive force or a resistance value according to the concentration of a specified component (for example, nitrogen oxide, oxygen, etc.) of exhaust gas is known. A stacked detection element manufactured by simultaneously sintering a plurality of stacked layers having different main components is also known. If the detection element is formed by simultaneous sintering, the sintering process is completed at one time, and a laminating process of layers is not required. Therefore, the number of processes can be remarkably reduced. For example, JP-A-2007-33374 or JP-A-2007-139749 describes a gas sensor having such a detection element.
JP-A-2007-33374 (FIG. 2 and related description in this reference) describes a gas sensor including a gas sensor element that includes a solid electrolyte layer configured to detect a specified gas component. A protection layer containing a main component different from the solid electrolyte layer is stacked on one surface of the solid electrolyte layer. Two insulating layers containing a main component different from the solid electrolyte layer are stacked on the other surface of the solid electrolyte layer. A total thickness of the two insulating layers is different from the protection layer.
The gas sensor element is manufactured as described below. That is, an unsintered solid electrolyte layer containing zirconia as a main component and becoming the solid electrolyte layer after sintering is formed. Two unsintered insulating layers containing alumina as a main component and becoming the insulating layers after sintering are formed. The unsintered insulating layers are stacked. An unsintered protection layer containing alumina as a main component and becoming a protection layer after sintering is formed. The unsintered solid electrolyte layer, the two unsintered insulating layers and the unsintered protection layer are stacked to form an unsintered stacked body. Thereafter, the unsintered stacked body is integrally sintered (simultaneously sintered) to obtain a gas sensor element.
JP-A-2007-139749 (FIG. 2 and related description in this reference) describes a gas sensor element including: a sensing portion including a first solid electrolyte layer and a second solid electrolyte layer, which are configured to detect a specified gas component; and an insulating layer stacked therebetween. A protection layer containing a main component different from a main component of the first and the second solid electrolyte layers is stacked on the second solid electrolyte layer. Two insulating layers containing a main component different from the main component of the first and the second solid electrolyte layers are stacked on the first solid electrolyte layer. A total thickness of the two insulating layers is different from the protection layer.
The gas sensor element is manufactured as described below. An unsintered first solid electrolyte layer containing zirconia as a main component and becoming the first solid electrolyte layer after sintering, and an unsintered second solid electrolyte layer containing zirconia as a main component and becoming the second solid electrolyte layer after sintering, are formed. An unsintered insulating layer containing alumina as a main component and becoming the insulating layer between the first solid electrolyte layer and the second electrolyte layer after sintering is formed. An unsintered protection layer containing alumina as a main component and becoming a protection layer after sintering is formed. Two unsintered insulating layers containing alumina as a main component and becoming an insulating layer after sintering are formed. The two unsintered insulating layers, the unsintered first solid electrolyte layer, the unsintered insulating layer, the unsintered second solid electrolyte layer and the unsintered protection layer are stacked in this order to form an unsintered stacked body. Thereafter, the unsintered stacked body is integrally sintered (simultaneously sintered) to obtain a gas sensor element.
However, the gas sensor element according to JP-A-2007-33374 and JP-A-2007-139749 may warp when sintered. This is because the shrinkage associated with sintering (hereinafter called a “sintering shrinkage”) of solid electrolyte layers containing zirconia as a main component is largely different from that of other layers containing alumina as a main component. If the gas sensor element becomes warped, the gas sensor element may be subject to cracking or breakage. This is because the gas sensor element contacts other members when the gas sensor is manufactured by mounting other members to the gas sensor element.
Accordingly, it can be considered that a difference in sintering shrinkage between the solid electrolyte layers and other layers is reduced by changing the materials of the respective layers that constitute the gas sensor element in order to prevent a sintered gas sensor element from warping. However, taking the performance, strength and durability of the gas sensor element into consideration, there is a restriction in changing the materials. Heretofore, it has been difficult to sufficiently reduce the difference in sintering shrinkage so as to prevent warping while still maintaining good sensor performance and physical characteristics.